The present invention relates to a hybrid circuit breaker being formed with a vacuum interrupter, a gas interrupter and a resistor which serves to suppress a transient recovery voltage generated when a circuit current is interrupted by circuit-opening of the breaker and also to suppress an overvoltage generated when the breaker is circuit-closed.
A circuit breaker is designed so as to prevent the occurrence of restriking at the time of current interruption and to ensure the state of current interruption. Accordingly, with respect to an overvoltage appearing in an electric power system, the system designer employing such a circuit breaker is required to consider only a transient recovery voltage generated at the time of current interruption.
For an electric power system with a rated voltage of 300 kV or less, the margin of dielectric strength is generally high enough. From this, an overvoltage due to the transient recovery voltage or an overvoltage caused by the breaker-closing can be restricted within a range of safety operation with respect to a coordination of insulation of the power system. Thus, no countermeasure is required to suppress an overvoltage caused by the opening or closing action of the circuit breaker.
In an electric power system of 500 kV rating, on the other hand, the coordination of insulation depends on the consideration of a cost performance or economy for the system construction, and the limit of dielectric strength of the system insulation (or the system insulation level) is generally set at about double the nominal operating voltage with respect to the ground potential. In view of this, a making resistor is adapted to a circuit breaker of a 500 kV power system so that an overvoltage caused by the breaker-closing is effectively damped or suppressed, thereby achieving the overvoltage suppression.
In an extremely high voltage (EHV) or ultra high voltage (UHV) electric power system of 700 kV to 1000 kV ratings, further critical consideration for the coordination of insulation is required. Thus, the system insulation level is set at 1.5 to 1.6 of the nominal operating voltage with respect to the ground potential. Such a system insulation level is too low from a practical view point. Unless a transient recovery voltage caused by the current interrupting action of an EHV/UHV breaker is effectively suppressed, not only when an overvoltage due to the breaker-closing appears but even when restriking is prevented from the breaker action, it is practically impossible to avoid potential overshooting beyond the system insulation level. This fact requires the use of a making/breaking resistor through which both the closing and opening breaker actions are effected.
However, since a surge impedance of the power system decreases with an increase in the system voltage, the value of a making/breaking resistor for suppressing the overvoltage of an EHV/UHV system has to be reduced. This requires a large heat (or power) capacity to the making/breaking resistor. Thus, the size of the making/breaking resistor body becomes bulky, and the share of volume or cost of the resistor body in the EHV/UHV breaker becomes prominently high.
Meanwhile, in recent years, SF.sub.6 gas interrupters are often utilized to a circuit breaker which is formed with a breaking resistor, a resistor commutating interrupter containing main contacts and a resistor current cutting-off interrupter containing resistor contacts, wherein each of these interrupters is actuated under prescribed controlled gas pressure. In such a circuit breaker, a current feeding period for the breaking resistor must be determined in accordance with both the arc time of the commutating interrupter and that of the cutting-off interrupter. This requires a further enlarged heat capacity to the resistor body.
FIG. 1 illustrates various voltage and current waveforms obtained under the rated gas pressure and under the interruption guaranteed gas pressure of a conventional SF.sub.6 gas-blast circuit breaker which is formed with a breaking resistor, a resistor current cutting-off SF.sub.6 interrupter connected in series to the breaking resistor and a resistor commutating SF.sub.6 interrupter connected in parallel to the series circuit of the breaking resistor and the cutting-off interrupter.
In FIG. 1, the symbol v0 denotes an electric power system voltage applied to the breaker, v1 denotes a potential difference between resistor contacts of the resistor current cutting-off interrupter, and v2 denotes an arc voltage appearing across the main contacts of the resistor commutating interrupter. Further, i0 denotes a breaker current to be interrupted by the breaker action and i1 denotes a current flowing through the breaking resistor. In the illustration of FIG. 1, the i0 phase deviates by about 90 degrees from the v0 phase. The symbol i0* denotes another breaker current whose phase deviation from v0 is smaller than the phase deviation of i0 from v0. The symbol SX denotes the stroke of the main contacts of the commutating interrupter, and SR denotes the stroke of the resistor contacts of the cutting-off interrupter. The symbol TAmin* indicates the minimum arc time of the main contacts under the rated gas pressure of the commutating interrupter. TAmin indicates the minimum arc time of the main contacts under the interruption guaranteed gas pressure. TAmax indicates the maximum arc time of the main contacts under the interruption guaranteed gas pressure. TRmin indicates the minimum arc time of the resistor contacts under the interruption guaranteed gas pressure. TRmax indicates the maximum arc time of the resistor contacts under the interruption guaranteed gas pressure. TQ indicates the maximum current feeding period for the breaking resistor.
In the following, discussion will be given to a case wherein a current commutation for the breaking resistor and an interruption of a fault current i0 are effected by the above-mentioned SF.sub.6 interrupters.
After delivering the interruption effecting power from a breaker driver (t0 in FIG. 1), both the commutating and cutting-off interrupters start to actuate (t1). Then, the main contacts of the commutating interrupter are opened (t2), and arcing appears at the main contacts. Such arcing disappears when breaker current i0 (or i0*) crosses the zero value (t3, t4 or t5). Suppose that the arcing of the main contacts disappears at time t5. Then, commutating interrupter is electrically circuit-opened and current i0 commutates to the breaking resistor (t5).
Thereafter, the resistor contacts of the cutting-off interrupter are opened (t6), and arcing appears at the resistor contacts. Such arcing disappears when the commutated resistor current i1 crosses the zero value (t7 or t8).
Generally speaking, the responsibility of the resistor commutating interrupter (main contacts) is more important than that of the resistor current cutting-off interrupter (resistor contacts). This is because, during the commutation, a large amount of fault current i0 must be interrupted and, in addition, the phase relation between voltage v0 and current i0 could be worse for this interrupting action (i.e., nearly 90 degrees phase difference could exist). For this reason, a large current handling capacity is required for the commutating interrupter.
On the contrary, the current handling capacity of the cutting-off interrupter may be far smaller than that of the commutating interrupter. This is because, the amount of current i1 to be interrupted by the cutting-off interrupter is far smaller than that by the commutation interrupter, and the phase of voltage v1 substantially matches the phase of current i1. However, when a small-current-capacity interrupter is employed for the cutting-off interrupter, the maximum period (TRmax) of a possible arc time of the resistor contacts is liable to exceed one cycle of current i1, as shown in FIG. 1.
Such a long arc time of the resistor contacts can be shortened if the current handling capacity of the cutting-off interrupter is enlarged. In this case, however, the mechanical power for driving or actuating the cutting-off interrupter is required to be further increased. (Conventionally, a power increase of 30 to 40% is required for the capacity-enlarged interrupter driving.) Then, the total size and cost of the circuit breaker body becomes large and, consequently, the manner of enlarging the current handing capacity of the cutting-off interrupter is not a good countermeasure.
Further, as exemplified in the illustration of FIG. 1, the minimum arc time TAmin* of the commutating interrupter (main contacts) under the rated gas pressure of, e.g., 6 kg/cm.sup.2 -g differs by roughly 0.2 cycles of current i0 from that TAmin under the interruption characteristic guaranteed pressure or interruption locking pressure of, e.g., 5 kg/cm.sup.2 -g. Such a minimum arc time difference (t3-t4) actually expands a possible current feeding period of the breaking resistor. Thus, when SF.sub.6 interrupters are conventionally applied to the commutating and cutting-off interrupters, roughly 2 cycles (t3-t8) of current i1 should be considered for the maximum current feeding period TQ of the breaking resistor. This results in prominently enlarging the necessary heat capacity for the breaking resistor body.